User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-14
This ALC voltage, and the DC level
corresponding to the IF level after signal
correction, are fed to U10A pin 2, whose
output at pin 1 connects to the ALC pin-
diode attenuator circuit, CR1-CR3. If
there is a loss of gain somewhere in an IF
circuit, the output power of the
transmitter will drop and the ALC circuit
will sense this drop at U10A that will
automatically lower the loss of the pin-
diode attenuator circuit that increases
the IF level through the attenuator
circuit.
The ALC action starts with the ALC
detector level that is monitored at TP4.
The detector output at TP4 is nominally
+.8 VDC and is applied through the
resistor R77 to a summing point at the
op-amp U10A pin 2. The current
available from the ALC detector is offset,
or complemented, by the current taken
away from the summing junction. In
normal operation, U10A pin 2, is at 0
VDC when the loop is satisfied. If the
recovered or peak-detected IF signal at
the IF input jack J7 of this board should
drop in level, which normally means that
the output power is decreasing, the null
condition, 0 VDC, would no longer occur
at U10A pin 2. When the level drops, the
output of U10A at pin 1, will go more
positive. If jumper W3 on J6 is in the
Automatic position, it will cause the ALC
pin-diode attenuators CR1, CR2, and CR3
to have less attenuation therefore
increasing the IF level through them that
will act to compensate for the original
decrease in level. If the ALC cannot
increase the input level enough to satisfy
the ALC loop, due to not enough range,
an ALC fault will occur. The fault is
generated because U10D pin 12,
increases above the trip point set by R84
and R83 until it conducts. This makes
U10D pin 14, high and causes the red
ALC Fault LED DS2 to light.
4.1.5.12 (Optional) Scrambled Operation
with Encoding
For optional encoded, scrambled
operation, the jumper W4 on J8, the
jumper W8 on J9, the jumper W7 on J26
and the jumper W5 on J21 must all be
between pins 2 and 3. The IF is
connected through W4 on J8 to the sync
regeneration circuits beginning with L37.
If this board is operated with scrambling,
using suppressed sync, the ALC circuit
operates differently than described above
because there is no peak of sync present
on the IF input. A timing pulse from the
scrambling encoder must connect to the
board at J24. This timing pulse is
converted to sync pulses by U17A and
U17B, which control the operation of Q8.
The sync amplitude is controlled by R149
and is then applied to U15A, where it is
added to the detected IF signal to
produce a peak of sync level. The output
of U15A is peak detected by CR26 and
fed to U15B. If necessary, the
intercarrier notch L39 can be placed in
the circuit by placing the jumper W6 on
J22. The intercarrier notch is adjusted to
filter any aural and 4.5-MHz intercarrier
frequencies. The peak of sync signal is
fed through R162, the ALC calibration
control, to amplifier U15C. The amplified
peak of sync output is connected through
J21 pins 2 and 3, to U10A, where it is
used as the reference for the ALC circuit
and the AGC reference to the transmitter
control board. Voltage TP4 should be the
same in either the normal or the encoded
video mode. Monitor J9 pins 3 and 4,
with a spectrum analyzer, check that the
board is in the AGC mode, and tune C103
to notch-out the aural IF carrier.
4.1.5.13 Mute Fault Command
NOTE: This fault command circuit is not
used in the 335B VHF transmitter.
The ALC board has circuitry for an
external mute fault input that can
connect to J19 pin 6. This is a Mute
command, in most systems, it is involved
in the protection of the circuits of high
gain output amplifier devices. The Mute
command is intended to protect the
amplifier devices against VSWR faults. In
this case, the action should occur faster